1. Field of the Invention
The invention relates to the field of electronics. More specifically, the invention relates to direct current buses (“DC buses”) used in power modules.
2. Background of the Invention
An inverter is commonly used to convert direct current (“DC”) to alternating current (“AC”) to power a three-phase load, such as a three-phase motor, or, alternatively, to convert AC from a three-phase source to DC. The inverter commonly contains six switches. Power modules often contain one or more pairs of complementary switches. The power module typically includes silicon dice on substrates that are secured to the module baseplate. Each switching pair has a positive or “high” side switch and a negative or “low” side switch for controlling the flow of electric current. Each switching pair is referred to herein as a “half bridge.” The “high side” of the bridge contains the positive switches, and the “low side” contains the negative switches. By the term “switch” is meant a switching device such as an insulated gate bipolar transistor (“IGBT”) or Bipolar Junction Transistor (“BJT”) or Metal Oxide Semiconductor Field Effect Transistor (“MOSFET”), either singly or in parallel.
Elements may be described herein as “positive” or “negative.” An element described as “positive” is shaped and positioned to be at a higher relative voltage than elements described as “negative” when the power module is connected to a power source. “Positive” elements are positioned to have an electrical connection that is connectable to the positive terminal of a power source, while “negative” elements are positioned to have an electrical connection that is connectable to a negative terminal, or ground, of the power source. Generally, “positive” elements are located or connected to the high side of the power module and “negative” elements are located or connected to the low side of the power module.
In a typical power module configuration, the high side switches are on one side of the module opposite the corresponding low side switches. A positive DC lead from a power source such as a battery is connected to a conducting layer in the high side of the substrate. Likewise, a negative DC lead from the power source is connected to a conducting layer in the low side of the substrate. The high side switches control the flow of current from the conducting layers of each high side substrate to output leads. Output leads, called “phase terminals” transfer alternating current from the three pairs of switches, or half bridges, to the motor.
Power modules typically have three half bridges combined into a single three-phase switching module, or single half-bridge modules that may be linked together to form a three-phase inverter. As would be understood by one of ordinary skill in the art, the same DC to AC conversion may be accomplished using any number of half bridges, which correspond to a phase, and each switching pair may contain any number of switching devices. For simplicity and clarity, all examples herein use a common three phase/three switching pair configuration. However, the invention disclosed herein may be applied to a power module having any number of switches.
Current flows from the power source through the positive DC lead to the conducting layer on the high side substrate. Current is then permitted to flow through one or more switching device on the high side to a conducting layer, commonly referred to as a phase output layer, on the low side. A phase terminal lead allows current to flow from this conducting layer on the low side to the motor. The current then flows from the motor to the corresponding conducting layer on the low side of a second switching pair through the low side switches and diodes to the negative DC lead to the power source.
Current flowing through various inductive paths within the module transiently stores energy which increases energy loss, reduces efficiency, and generates heat. When the flow of current changes, as in such a high frequency switching environment, large voltage overshoots often result, further decreasing efficiency. In addition, the DC terminals are commonly attached to one end of the power module, which forces current to travel further to some switches for some switching configurations, than for others, resulting in non-uniform current loops. Current loops that are not uniform result in uneven or inefficient motor performance. Additional materials regarding efficient configurations of power modules may be found in application Ser. No. 09/957,042, entitled “Press (Non-soldered) Contacts for High Current Electrical Connections in Power Modules,” application Ser. No. 09/957,001, entitled “EMI Reduction in Power Modules through the Use of Integrated Capacitors on the Substrate Level,” and application Ser. No. 09/882,708 entitled “Leadframe-based Module DC Bus Design to Reduce Module Inductance” filed Jun. 15, 2001, which are hereby incorporated by reference in their entirety.
These and other problems are avoided and numerous advantages are provided by the device described herein.